1. Field of the Invention
This invention relates to linear analog circuits suitable for applications such as voltage-to-current-converters, differential coupling stages, wideband squarers, rectifiers and the like. More particularly, this invention relates to analog circuits basically comprising four transistor junctions with cross-over collector-to-base interconnections between two of the transistors. Such circuits have been referred to as cross-quad circuits.
2. Description of the Prior Art
The circuits to be described herein are the type which make use of the linear relationship between the transconductance of a bipolar transistor and the collector current (thus giving rise to the name "translinear" circuits). Stated in an equivalent form, these circuits are based on the linear relationship between the base-emitter voltage and the logarithm of collector current. This linear relationship holds over many orders of magnitude for most present day small-signal transistors, and serves as the basis for a number of "non-linear" analog devices such as multipliers, dividers, squarers, rooters, and so on. The so-called translinear circuits are not confined to use in non-linear applications, and can for example provide a means for achieving very wide-band linear amplification. One of the important virtues of translinear circuits is in providing temperature-insensitive performance.
Translinear circuits operate entirely in the current domain (except for the ultimate interface with standard voltage-mode equipment), and are thus distinguishable from circuits the functioning of which involves a translation between V.sub.BE and collector current, requiring temperature compensation. Such voltages as do arise in a translinear circuit due to signals are very small (tens of millivolts usually) and of only incidental importance to operation of the circuit. Analytic procedures can be employed completely ignoring such voltages. The resulting functions are fundamentally exact algebraic expressions in which the basic device parameters do not appear explicitly, and which are insensitive to isothermal temperature variations over a wide range.
One basic aspect of a translinear circuit is that it involves the use of an even number of forward biased PN junctions arranged in loops around which the net voltage sums to zero, that is, there are as many junctions connected in one polarity as in the other. Many circuits have only a single loop, but loops may overlap, The PN junctions do not have to be the emitter-base junction of a bipolar transistor; in theory, two-terminal devices may be used, although in practice their accuracy may be inadequate.